Nations are falling behind in efforts to slow climate change, the UN warns in a draft report, and must dramatically reduce carbon emissions in the next 15 years. That is proving difficult as the world's major economies rebound from a global recession.

Steam billows from the cooling towers of Vattenfall's Jaenschwalde brown coal power station behind wind turbines near Cottbus, eastern Germany. World leaders have 15 years to decarbonize their energy supplies before the risks of climate change become intractable, according to a newly leaked draft UN report.

For a while, it looked like some of the world's biggest economies might actually decouple development from ballooning energy-related greenhouse gas emissions – eeking out growth while lowering their carbon output or at least keeping it in check.

But with the global economy slowly rebounding from the Great Recession, the developed world's carbon emissions are doing the same. And they're rising as warnings about the risks of burning carbon-heavy fuels become more urgent. World leaders have 15 years to decarbonize their energy supplies before the risks of climate change become intractable, according to a newly leaked draft report from the United Nations.

The cost of renewables continues to drop, and public support for lowering carbon emissions remains strong. An unexpected technological breakthrough could rapidly accelerate a transition to clean energy. But, for now, emissions show no signs of slowing, even in places where efforts to decarbonize are strongest.

Energy policies around the world offer too much support for oil, gas, and coal, and do too little to encourage a transition to wind, solar, and other renewable energies, according to the draft UN report, which was leaked to The New York Times and Reuters.

If nothing is done, it reportedly reads, “the fundamental drivers of emissions growth are expected to persist despite major improvements in energy supply."

Climate change warnings from the UN's Intergovernmental Panel on Climate Change are not new, but the latest alert comes as decarbonization efforts across the globe run into trouble.

After peaking in 2007, US carbon emissions dropped for four out of six years. Much of that was the result of a collapsing economy, but it also came as environmental regulations and a boom in cleaner-burning US natural gas took hold. By 2012, emissions in the world's largest economy had dropped to the lowest they've been since the mid 1990s, despite modest gains in gross domestic product.

Last year was a different story. Emissions rebounded 2 percent as the US economy continued to show signs of recovery. They are projected to rise slightly again in 2014 and 2015 as natural gas prices rise and coal regains market share. The US can still meet its goal of 17 percent reduction in emissions from the 2005 level by 2020, but it will require some changes in the energy sector.

Germany has laid out an aggressive plan to shift to renewables and cut carbon emissions. The world's fourth-largest economy has tripled its renewables capacity since 2000 and emissions are well below 1990 levels. But carbon emissions ticked up in 2012, and appear to have done the same in 2013.

Much of that is from a rebound in coal use, with Germany burning more coal in 2013 than it has in a year since 1990, according to preliminary figures. Germany aims to reform its renewables law later this year to address these adverse effects, along with rising retail electricity costs.

Others like the United Kingdom and Australia are second-guessing or altogether abandoning carbon taxes. The UK's greenhouse gas emissions jumped 3.5 percent in 2012, bucking a longer downward trend. Emissions in Australia jumped 3 percent in 2011 to an all-time high.

And these are all in the developed world. The real future growth in emissions, according to most projections, will come from China, India, and myriad countries fueling industrialization with carbon-heavy coal.

“The continued and dangerous rise in greenhouse gases in the atmosphere is in large part the direct result of past investments in energy and mobility systems based on the use of fossil fuels,” Christiana Figueres, the UN's top climate chief, told investors at the 2014 Investor Summit on Climate Risk Wednesday, according to a statement. “New investments must now assist in reversing this unsustainable trend, and quickly if the world is to have a chance of staying under a 2 degree Celsius temperature rise.”

To meet global climate goals, investments in clean energy and energy efficiency need to reach about $1 trillion per year, according to Ms. Figueres. Last year, those investments fell for the second year straight, according to Bloomberg New Energy Finance, dropping 11 percent to $254 billion.